Efficient usage of a multi-level register file utilizing a register file bypass

ABSTRACT

A processor includes an execution unit, a first level register file, a second level register file, a plurality of storage locations and a register file bypass controller. The first and second level register files are comprised of physical registers, with the first level register file more efficiently accessed relative to the second level register file. The register file bypass controller is coupled with the execution unit and second level register file. The register file bypass controller determines whether an instruction indicates a logical register is unmapped from a physical register in the first level register file. The register file controller also loads data into one of the storage locations and selects one of the storage locations as input to the execution unit, without mapping the logical register to one of the physical registers in the first level register file.

BACKGROUND

Embodiments of the inventive subject matter generally relate to thefield of computer architecture, and, more particularly, to amulti-threaded processor having multiple levels of register files.

As processors have evolved, the number of simultaneous threads theysupport has increased. In order to support this increase, the number ofregisters in a processor has also increased. This, along with anincrease in data word size (e.g., 32 bits to 64 bits) and other factors,has led to larger register sets. As register sets get larger, dataaccess time (latency) and power consumption has also increased.

In an effort to deal with this increase in latency and powerconsumption, several techniques have been employed, including usingmirrored sets of registers that may be accessed independently, andadopting multi-level register files. Multi-level register files providea smaller register file with lower latency and lower power consumptionfor primary usage, and a larger register file with higher latency andpower consumption for secondary usage.

SUMMARY

Embodiments of the inventive subject matter include a method in whichdata bypasses a first level register file. A processor receives anindication that data has been selected to be moved from a first levelregister file to a second level register file. The processor determinesthat an unissued instruction references the data selected for movingfrom the first level register file to the second level register file. Inresponse to this determination, the processor indicates that theunissued instruction references the data selected for moving from thefirst level register file to the second level register file. Theprocessor determines that a set of storage locations, which aredesignated for bypassing register mapping, may be used to host the datafrom the second level register file. In response to this determination,the processor writes the data from the second level register file to oneof the storage locations and indicates that the data referenced by theunissued instruction is located in one of the storage locations.

Embodiments of the inventive subject matter include a method where datais loaded into a storage location designated for bypassing registermapping. The processor determines that a logical register has beenselected to be unmapped from a physical register in the first levelregister file. The unmapping process moves the data from the physicalregister in the first level register file to a physical register in asecond level register file. The processor determines that an instructionin the issue queue indicates the logical register, and loads the datainto the first of a set of one or more storage locations designated forbypassing register mapping. The processor then selects the storagelocation the data was loaded into as input to an execution unit thatwill execute the instruction. The execution unit executes theinstruction with the data while bypassing register mapping of thelogical register.

Embodiments of the inventive subject matter include a processor with anexecution unit, first level register file, second level register file,plurality of storage locations and a register file bypass controller.The first and second level register files are comprised of physicalregisters, and the first level register file is more efficientlyaccessible relative to the second level register file. The plurality ofstorage locations are designated to host data associated with logicalregisters that will bypass register mapping. The register file bypasscontroller is coupled with the execution unit and the second levelregister file of physical registers. The register file bypass controllerdetermines whether an instruction indicates a logical register unmappedfrom a physical register in the first level register file before theinstruction issues. The processor also loads data corresponding to thelogical register into one of the plurality of storage locations, andselects one of the plurality of storage locations as input to theexecution unit for execution of the instruction. The instruction isexecuted without mapping the logical register to one of the physicalregisters in the first level register file.

BRIEF DESCRIPTION OF THE DRAWINGS

The present embodiments may be better understood, and numerous objects,features, and advantages made apparent to those skilled in the art byreferencing the accompanying drawings.

FIG. 1 depicts an example conceptual diagram of selected components of amicroprocessor system comprising a multi-level register file andregister file mapper with a performance-based level selecting registerfile controller.

FIG. 2 depicts a flowchart of example operations for determining whetheran instruction should flow into a first level register file or a lowerlevel register file.

FIG. 3 depicts an example diagram of selected components of themicroprocessor system comprising a multi-level register file, registerfile mapper, and register file bypass.

FIG. 4 depicts a flowchart of example operations for determining ifinstructions in the instruction queue reference a cast out register tag.

FIG. 5 depicts a flowchart of example operations for utilizing aregister file bypass.

FIG. 6 depicts the states of an example register file mapper implementedusing a hasher after a sequence of instructions.

FIG. 7 depicts an example computer system with a performance-based levelselecting register file controller.

FIG. 8 depicts an example register usage pattern graph.

DESCRIPTION OF EMBODIMENT(S)

The description that follows includes exemplary systems, methods,techniques, instruction sequences and computer program products thatembody techniques of the present inventive subject matter. However, itis understood that the described embodiments may be practiced withoutthese specific details. For instance, although examples refer to using afirst level register file consisting of one set of registers and asecond level register file consisting of multiple banks of registers,embodiments are not so limited. Many different multi-level register fileconfigurations are possible, including a first level register fileconsisting of multiple banks of registers, multiple first level registerfiles consisting of one set or multiple banks of registers, a secondlevel register file with only one set of registers, multiple secondlevel register files, and additional levels of register files, amongmany other variations. Furthermore, although embodiments may describe aset of scenarios where certain functionality may be used, thefunctionality is not limited to the described scenarios, nor is thefunctionality required to be used in the described scenarios. In otherinstances, well-known instruction instances, protocols, structures andtechniques have not been shown in detail in order not to obfuscate thedescription.

A processor operates on data dictated by instructions that specify theoperation (such as addition or subtraction), the locations of theoperands, and the location where the result should be placed. Thelocations of the operands are references to registers, which are storagelocations in the processor. An example of an instruction might be: ADDR1, R2, R3. This instruction tells the processor to add the values inregister one (R1) and register two (R2), and place the result intoregister three (R3). “ADD” is the operation, “R1” and “R2” are operandlocations, and “R3” is the result destination. The operation is referredto as an “opcode,” while operand locations are referred to as “sourceregisters” and the result destination as the “destination register.”

An Instruction Set Architecture (ISA) defines the availableinstructions, number of registers, and other specifics. The ISA maylimit the number of registers available to a certain number, but thislimitation does not limit the hardware itself. Therefore, the ISA maydefine registers one through thirty-two, but the processor itself maycontain sixty-four registers. The registers defined by the ISA arecalled “architectural registers,” and the registers that the processoractually contains are called “physical registers.” The “architectural”registers are logical, and will be referred to herein as “logicalregisters.” Thus, the source registers specified in an instruction are“logical source registers” and the physical registers where the dataresides are the “physical source registers.” Destination registers maybe “logical destination registers” and “physical destination registers.”Additionally, the physical registers are organized into multipleregister files.

In addition to potentially containing a larger number of physicalregisters than logical registers, registers may be grouped into multipleregister files. One way to group these register files is into multiplelevels. For instance, a first level register file (FLRF) has thesmallest size, providing it with the lowest latency and thus higherperformance. Subsequent levels of register files are larger in size,thus increasing latency and decreasing performance. The size of registerfiles may be increased by including multiple banks of registers. Forexample, a second level register file (SLRF) may contain four banks ofregisters, with each bank containing the same number of registers as theFLRF.

Instructions go through several stages before getting executed.Initially, the instruction fetch unit (IFU) fetches an instruction fromthe instruction cache. The IFU decodes the instruction to determine theopcode and logical source registers, if any. The IFU then moves theinstruction to the instruction dispatch unit (IDU). Prior to dispatchingthe instruction, the IDU determines if the one or more logical registersspecified by the instruction has been renamed or mapped to the physicalregisters. Register renaming for logical and physical registers will bereferred to herein as register mapping or mapping to distinguish fromregister renaming that liberates code from sequential processing. If amapping for the logical register does not exist, a swap is requested andthe IDU dispatches the instruction to the instruction queue (IQ). TheIDU notifies the IQ that the instruction is waiting on a swap so thatthe IQ will not issue the instruction until the swap is complete. When aswap is requested, the processor swaps the contents of a physicalregister(s) in a lower level register file into a physical register(s)of an FLRF. Swapping also involves updating a mapping structure toreflect the swap of data from the lower level register file physicalregister(s) into the first level register file physical register(s). TheIQ is then notified of the FLRF physical register(s) into which theoperands were loaded. If a mapping already exists, the IDU dispatchesthe instruction to the IQ, allowing the instruction to proceed withoutstalling. The IQ sends the instruction to the execution unit when alloperands are present in the FLRF. If the instruction designates alogical destination register, a physical register is allocated bycreating a mapping between the logical destination register and thephysical register in the FLRF. The result of the execution is stored inthe mapped physical register.

For various reasons, an instruction may be in an incomplete state. Theseinstructions are referred to as in-flight instructions. Executedinstructions that are in a complete state, the result(s) of which may beused by subsequent instructions, are referred to as completedinstructions. In-flight instructions may have data stored in thephysical registers along with results of completed instructions.

A processor microarchitecture includes one or more components to managethe mapping of logical registers to physical registers of multiplelevels of register files. This description refers to these one or morecomponents as a register file mapper. A register file mapper can beimplemented to include a register file mapper for in-flight instructions(“in-flight register file mapper”) and a register file mapper forcompleted instructions (“completed register file mapper”). The IDU sendsan instruction or fields of the instruction to the register file mapperto determine whether the logical registers referenced by theinstruction, if any, are mapped to the physical registers. If theinstruction indicates a logical destination register, the register filemapper allocates a physical register for the instruction and records anindication of the allocation in the in-flight register file mapper. Ifthe instruction does not reference any logical source registers, theregister file mapper signals the IDU to dispatch the instruction. If theinstruction does reference logical source registers, the register filemapper determines if an entry exists for the logical source registers inthe register file mapper. If an entry exists for all referenced logicalsource registers of an instruction, the register file mapper signals theIDU to dispatch the instruction to the issue queue. If one or moresource logical registers do not have an entry in the register filemapper, the register file mapper attempts to read the referenced datafor the source logical registers from the SLRF. The register file mapperthen writes it to the FLRF, swapping it with other data if necessary.When data is written to the FLRF, the register file mapper inserts orupdates an entry in a register mapping structure, recording indicationsof the source logical register, instruction thread identifier, andregister tag. If the data was not in the SLRF, the register file mapperrequests it from the memory hierarchy and updates the entry in theregister file mapper accordingly.

Because the number of registers in a processor may be insufficient tohold all data the processor may use, space in the FLRF may be at apremium. Additionally, swapping data between the FLRF and SLRF, as wellas retrieving data from the memory hierarchy requires extra time.Reducing swaps and memory accesses increases efficiency of the registerfile mapper and multiple register files, which increases processorperformance. Managing the destination of data within the multi-levelregister files based on the potential impact the destination may have onfuture instructions assists in using the multi-level register filesefficiently. When an instruction completes, the result would typicallygo into the FLRF, and the register file mapper would write thecorresponding register mappings into the completed register file mapper.If a conflict arose in which another thread needed to access the samelogical register or the space was otherwise needed, the completedregister file mapper would have to decide which data to cast out to theSLRF to solve the conflict or free space. This data movement potentiallyputs data that will be used by a subsequent instruction in the higherlatency SLRF, leading to decreased performance.

A register file mapper can be designed to write particular data to theSLRF without writing an entry into the completed register file mapper.If there is no entry in the completed register file mapper for aparticular physical register, then that physical register in the FLRF isavailable. This allows the lower latency FLRF to be used for data thatmore likely impacts performance, reducing the likelihood of performanceimpacting cast outs of data from the FLRF to the SLRF.

FIG. 1 depicts an example conceptual diagram of selected components of amicroprocessor system comprising a multi-level register file andregister file mapper with a performance-based level selecting registerfile controller (hereinafter referred to as just “register filecontroller”). FIG. 1 depicts a microprocessor system 100 including anIFU 101, IDU 103, IQ 105, execution units 107, a register file mapper102, an FLRF 140, an SLRF 150, and an optional set of latches 161 andoptional multiplexer 163. The register file mapper 102 contains anin-flight register file mapper 110, a completed register file mapper120, and a register file controller 129. The completed register filemapper 120 includes a map table 121. The map table includes entries 122.Each of the entries 122 includes a logical register identifier field123, a thread identifier field 125, and a register tag field 127. Thelogical register identifier field 123 and the thread identifier field125 host identifiers that act as indexes into the map table 121. Thein-flight register file mapper 110 includes a map table similar to themap table 121. The FLRF 140 contains a plurality of physical registers141. The physical registers 141 of the FLRF 140 are identified by thevalues in the register tag fields 127 of the map table 121. The SLRF 150includes register file banks 153. Each bank can include the same numberof registers as in the FLRF 140.

When the execution unit 107 completes execution of an instruction thatreferences a logical destination register, the execution unit 107notifies the register file controller 129. Before completion, an entryin the in-flight mapper 110 for the instruction indicates a logicaldestination register identifier, a register tag, and a threadidentifier. The logical destination register identified by the logicaldestination register identifier has previously been mapped to a physicalregister identified by the register tag. The execution unit 107 writesthe result of executing the instruction into the one of the physicalregisters 141 identified by the register tag. The register filecontroller 129 writes the register tag, the instruction threadidentifier and logical destination register identifier from thein-flight mapper 110 into the map table 121. The register filecontroller 129 then removes the entry for the executed instruction fromthe in-flight register file mapper 110.

A physical register is considered allocated when an entry with thatphysical register's register tag exists in the in-flight register filemapper 110 or the map table 121. If all of the physical registers 141are allocated and data not currently contained in the FLRF 140 isneeded, then data is cast out from the FLRF 140 or swapped between theFLRF 140 and the SLRF 150. In order to cast out data from the FLRF 140,the register file controller 129 creates a cast out request, putting itinto a queue with swaps, loads, and other operations that use theregister file read and write ports. When the cast out request is ready,the register file controller 129 writes the data to the SLRF 150 andremoves the mapping from map table 121. This entire process will behereinafter referred to as “casting out” (and related variations).

In certain scenarios, the register file controller 129 can determinethat the result of an instruction may be cast out from the FLRF 140 tothe SLRF 150 immediately. This deallocates the mapped physical registerand makes it available for data that has a greater impact onperformance. These scenarios occur when the access latency for oneinstruction result may be increased in order to decrease the accesslatency for other data. The register file controller 129 can usemetadata associated with an instruction to help determine whether aninstruction result may be cast out immediately. The metadata may take avariety of forms, the most common being flags associated with theinstruction, special opcodes, and statistics kept by the processor orthe register file controller 129.

For example, the register file controller 129 can be designed to check aflag to determine if an instruction is associated with a low prioritythread. If the thread has a low priority, then casting out associateddata provides more space for data associated with higher prioritythreads. The register file controller 129 can also be designed to checka flag to determine if an instruction is associated with a microcodedroutine. A microcoded routine is created when the processor breaks acomplex instruction up into simpler instructions. The results from thesimpler instructions are typically consumed very quickly, thus theresult will likely be consumed before the cast out request gets throughthe register file controller 129 queue. The register file controller 129can also be designed to check a flag to determine if an instruction isassociated with a software routine that does not typically consumeinstruction results for a long period of time, if at all. For example,an initialization routine may load data that might be used at someunknown point in the future. This data may be cast out immediately,allowing the physical registers 141 to be used for data that may be usedsooner.

Additionally, the register file controller 129 can be designed to trackthe frequency of logical register usage or the thrashing rate of athread. The register file controller 129 can cast out infrequently usedlogical destination registers immediately because the probability thedata will be accessed soon is small. When a thread is thrashing, thelogical destination registers are not being accessed frequently. Thiscauses many swaps, increasing the conflicts for the physical registers141. When the register file controller 129 casts out the logicaldestination registers associated with an excessively thrashing thread,swaps are reduced, increasing performance.

To track the frequency of logical register usage, the register filecontroller includes a counter register for each logical register and aspecial register that holds a programmable threshold value. When alogical register is accessed, the associated counter register isincremented. When an instruction result is stored in a logical register,the register file controller 129 compares the value in the associatedcounter register with the threshold value. If the counter register valueis smaller than the threshold value, the register file controller 129determines that the logical register is infrequently used. The counterregister is reset after a certain number of clock cycles.

To track the thrashing rate of a thread, the register file controller129 includes a counter register and a programmable threshold registerfor each thread. When data associated with a thread was cast out orswapped, the associated counter register is incremented. When aninstruction result is stored in a logical register, the register filecontroller 129 compares the value in the associated counter registerwith the threshold value. If the counter register value is larger thanthe threshold value, the register file controller 129 determines thatthe thread is excessively thrashing. The counter register would be resetafter a certain number of clock cycles.

In these scenarios, when the execution unit 107 completes execution ofan instruction that references a logical destination register, theexecution unit 107 notifies the register file controller 129. Theexecution unit 107 then writes the result into the one of the physicalregisters 141 identified by the register tag mapped to the logicaldestination register identifier as specified in the in-flight registerfile mapper 110. If the register file controller 129 determines that theresult can be cast out immediately, the register file controller 129removes the entry from the in-flight register mapper 110. But theregister file controller 129 does not insert an entry into the map table121. Instead, the register file controller 129 casts out the result fromthe physical register referenced in the in-flight register file mapper110, writing the result into the SLRF 150. Because no entry exists inthe map table 121 or the in-flight register file mapper 110 for theregister tag identifying the physical register, the physical registerbecomes available for other data immediately. This avoids occupying thephysical register in the FLRF 140 with data that will not be consumedsoon, and either avoids evicting data that will be consumed soon orallows the physical register to remain available for other data.

FIG. 2 depicts a flowchart of example operations for determining whetheran instruction should flow into a first level register file or a lowerlevel register file. As an example flowchart, FIG. 2 presents operationsin an example order from which embodiments can deviate (e.g., operationscan be performed in a different order than illustrated and/or inparallel).

At block 201, a register file controller determines if an instruction isassociated with a low priority thread. The register file controllerchecks a flag associated with the thread or the instruction itselfspecifies priority. The ISA may include this flag, allowing it to be setexplicitly by the programmer or compiler. It may also be determineddynamically by the operating system, processor, or other software orhardware mechanism. If the register file controller determines that theinstruction has been given a low priority, then control flows to block230. If the register file controller determines that the instruction hasnot been specified as low priority, then control flows to block 203.

At block 203, a register file controller determines if an instruction ispart of a microcoded routine. A microcoded routine is one instructionthat is broken up by the processor into multiple, simpler instructions.The program typically consumes the results of these instructions verysoon after completion, so they can be cast out quickly while havinglittle impact on performance. The processor generates the microcodedroutine, thus allowing a flag associated with the instruction or specialopcodes to signify that the instruction is part of a microcoded routine.If the register file controller determines that the instruction is partof a microcoded routine, then control flows to block 230. If theregister file controller determines that the instruction is not part ofa microcoded routine, then control flows to block 205.

At block 205, a register file controller determines if the logicaldestination address referenced by the instruction is an infrequentlyused logical register. Software compilers tend to use the beginning andending register ranges more frequently than others. Thus, some logicalregisters are used much less frequently than others. If a logicalregister is used infrequently, there is a high likelihood that it willnot be referenced again for some time, and the associated data may becast out, allowing more frequently used data to be stored in thephysical register. Logical register usage frequency is determined bycomparison with a static list or dynamically. A static list can becreated by running hardware simulations and determining which availablelogical registers are used infrequently. The hardware designers then putthe identifiers of the logical registers into the register filecontroller or other accessible location, allowing the register filecontroller to compare the logical destination register to the list. Todetermine infrequently used logical registers dynamically, the registerfile controller may have a rate counter that increments on each read,write, or swap of the logical register. The rate counter would resetafter a certain number of clock cycles. If the rate counter was under aprogrammable threshold for the referenced logical destination register,the logical register is infrequently used. If the register filecontroller determines that the instruction references an infrequentlyused logical register, then control flows to block 230. If the registerfile controller determines that the instruction does not reference aninfrequently used logical register, then control flows to block 207.

At block 207, a register file controller determines if the threadassociated with the instruction is being frequently cast out of theregister file mapper. This is referred to as “thrashing.” If a thread isthrashing, it is not referencing the logical registers it is usingfrequently, potentially forcing data for other threads to be cast outunnecessarily. If the register file controller detects that a thread isthrashing, it may cast out any instruction results of the thrashingthread from the register file mapper to the SLRF incident to determiningthat the thread is thrashing. This prevents conflicts with otherthreads, resulting in fewer swaps. To determine if a thread isthrashing, the register file controller may keep a rate counter perthread that is incremented each time a particular thread has a logicalregister cast out. The rate counter would reset after a certain numberof clock cycles. If the rate counter reached a programmable threshold,the register file controller would determine that the thread wasexcessively thrashing. If determined to be excessively thrashing, theregister file controller would immediately cast out the results fromcompleted instructions associated with that thread. Control would thenflow to block 230. If the register file controller determined the threadwas not thrashing, control would flow to block 209.

At block 209, a register file controller determines if the softwareroutine running is merely initialized or cleaned up at completion.Certain software routines, such as interrupt handlers, run once and donot access the results after instructions are completed. In theseinstances, the register file controller may cast the results out sincethey are unlikely to be referenced again soon. These software routinesmay communicate this status to the processor by utilizing asoftware-accessible register coupled to the register file controller.The software-accessible register could be a register included in theprocessor separate from the FLRF, or could be a register in the FLRFreserved for this purpose. The software routine would set a specifiedbit in the register to notify the processor or register file controllerthat the data may be cast out immediately. If the register filecontroller determined the results may be cast out immediately, controlwould then flow to block 230. If the register file controller determinedthe results may not be cast out immediately, then control would flow toblock 240.

At block 230, the register file controller determines if the logicaldestination register identifier of the instruction has been mapped to aphysical register. If the logical destination register has been mappedto a physical register, then control flows to block 235. If the logicaldestination register has not been mapped to a physical register, thencontrol flows to block 231.

At block 231, the register file controller removes the logicaldestination register identifier from the in-flight register file mapper.No mapping is created in the completed register file mapper for thelogical destination register identifier. Control then flows to block233.

At block 233, the register file controller casts the result in themapped physical register out, writing it to the SLRF. Because a mappingfor the logical destination register existed only in the in-flightregister file mapper, no mapping in the completed register file maptable is removed.

If the logical destination register was already mapped to a physicalregister, then control flowed from block 230 to block 235. At block 235,the register file controller sets the logical destination registermapping to “least recently used.” Each mapping has a value associatedwith it indicating how much time has passed since the mapping was used.This value is referred to as the “age” of the mapping and may be storedin a special set of registers or with the mapping itself. When using a“least recently used” algorithm, the oldest mapping is the first to becast out or swapped. To set a particular mapping to “least recentlyused,” the age of the mapping is set to be the oldest. Setting themapping to “least recently used” allows the result to remain in theFLRF, but set to be the next swap or cast out victim.

If none of the criteria for selecting the SLRF instead of the FLRFapply, then control flowed to block 240 from block 209. At block 240,the register file controller removes the logical destination registeridentifier from the in-flight register file mapper. Control then flowsto block 243.

At block 243, the register file controller writes the logicaldestination register identifier, associated thread identifier, andregister tag into the completed register file mapper map table.

In some scenarios, the register file controller does not have a resultthat satisfies a criterion for being written to the SLRF instead of theFLRF. Data likely to be consumed soon may occupy all of the physicalregisters in the FLRF. Thus, casting out data in any one of the physicalregisters of the FLRF will impact performance. For example, swaps mayonly be available at instruction dispatch. Therefore, any datareferenced by an instruction in the IQ cannot be cast out withoutanother mechanism to allow access to the data. If the ability of theregister file controller to cast out data is limited, the ability of theregister file controller to optimize the use of the FLRF is alsolimited. To allow the register file controller to cast out datareferenced by instructions in the IQ, a register file mapper can bedesigned to include a “register file bypass.” The register file bypassallows an execution unit to bypass a higher level register file and readdirectly from a lower level register file. In addition, the directaccess provided by the register file bypass ameliorates the performanceeffect of casting out of data on which a subsequent instruction depends.The register file mapper can then cast out data from the higher levelregister file, even if an instruction in the IQ references that data.

When the register file mapper casts out data, the register file mappernotifies the IQ of the associated register tag. The IQ then scans theinstructions in the queue, setting a flag for any that reference theregister tag. The IQ can then speculatively issue an instruction withthe flag set while sending the register file bypass controller aregister file bypass request. If the register file bypass is permitted,the register file bypass controller loads the requested data into aregister file bypass storage location, which may be a special registeror latch. Once loaded into a register file bypass storage location, thedata is available for the execution unit. If the register file bypassrequest is denied, the execution unit rejects the instruction, which maybe reissued again at a later point.

FIG. 3 depicts an example diagram of selected components of themicroprocessor system comprising a multi-level register file, registerfile mapper, and register file bypass. FIG. 3 depicts a subset of amicroprocessor system 300, including an IQ 305, execution units 307, aregister file controller 329, an FLRF 340, an SLRF 350, a register filebypass controller 365, and a set of latches 361 and a multiplexer 363.The FLRF 340 contains a plurality of physical registers 341 identifiedby register tags. The FLRF 340 may also include an optional set ofphysical bypass registers 347. The SLRF 350 includes register file banks353. A thread identifier and logical register identifier associated withthe data in each latch 361 is tracked by the register file bypasscontroller 365. Each physical register in the set of physical bypassregisters 347 also has a thread identifier and logical registeridentifier associated with it. These identifiers can be located in theregister file bypass controller 365 or other component accessible to theregister file bypass controller 365.

Instructions in the IQ 305 include register tags and a “needs_rtag” flagfor each logical source register. If not all logical source registersare available at dispatch, a swap request is created by the IDU. Theinstruction may then be dispatched to the IQ 305 with the “needs_rtag”flag set for the logical source registers for which the swap request wascreated. This flag alerts the IQ 305 that the associated data ismissing, keeping it from issuing the instruction until the data isavailable in the FLRF 340.

When all available physical registers 341 are occupied by data, theregister file controller 329 may determine that data in one of thephysical registers 341 should be cast out to the SLRF 350. After makingthis determination, the register file controller 329 notifies the IQ 305of the register tag for the physical register containing the data to becast out. The IQ 305 sets the “needs_rtag” flag for any instruction thatrefers to the register tag and is associated with the same thread. TheIQ 305 may then speculatively issue any instruction that is not waitingon a swap, provided sources with the “needs_rtag” flag off areavailable.

In some embodiments, the microprocessor system 300 has latches 361 withan output port 367 connecting the SLRF 350 with the latches 361.Microprocessor system 300 also has a multiplexer 363 with inputs 369from the FLRF 340, inputs from latches 361, and an output 371 connectedto execution units 307. The thread identifier and logical registeridentifier associated with the data in each latch 361 is tracked by theregister file bypass controller 365. If the IQ 305 speculatively issuesan instruction with a “needs_rtag” flag on, the register file bypasscontroller 365 compares the instruction thread identifier and unmappedlogical register identifier with the thread and logical registeridentifiers associated with the latches 361. If the needed data ispresent in one of the latches 361, the register file bypass controller365 sets the multiplexer 371 to select the input from the appropriateone of latches 361. The value in the selected latch is provided to oneof the execution unit inputs. If the data does not exist in the latches361, the register file bypass controller 365 initiates a register filebypass request, discussed below.

In some embodiments, microprocessor system 300 has physical registers inthe FLRF 340 designated as bypass registers 347. In one variation ofthis embodiment, the bypass registers 347 share an SLRF read port 373with the physical registers 347. Although the bypass registers 347 arepart of the FLRF 340, the bypass registers 347 are not available to theregister file mapper, unlike the physical registers 341. The registerfile bypass controller 365 tracks the data present in the bypassregisters 347 using the associated thread identifier and logicalregister identifier. The register file bypass controller 365 may alsouse other identifying information, or may update the source register tagto be one of the bypass register tags. If the IQ 305 speculativelyissues an instruction with a “needs_rtag” flag on, the register filebypass controller 365 determines if the data needed is in one of thebypass registers 347. If so, the register file bypass controller 365sets multiplexer 371 to provide one of the execution units 307 the datafrom the appropriate bypass register. If the data does not exist in thebypass registers 347, the register file bypass controller 365 initiatesa register file bypass request, which is discussed below.

The register file bypass controller 365 arbitrates between multiplebypass requests, deciding which to accept. There is limited bandwidthbetween the SLRF 350 and the execution units 307, thus the register filebypass can handle only a finite number of register file bypass requests.In the embodiments utilizing bypass registers 347, the bypass registers347 share an SLRF read port 373 with the physical registers 341. Thisread port is used by swaps in addition to register file bypasses, thusthe register file bypass controller 365 may deny register file bypassrequests to allow swaps to complete. Additionally, it is possible thatthe cast out request has not completed yet, so the data is not in theSLRF 350. In this case, the register file bypass request is denied sincethe data cannot be loaded.

If a register file bypass request is accepted, the register file bypasscontroller 365 moves the requested data into the particular storagemechanism used, whether a latch or bypass register. The register filebypass controller 365 may determine the location to use based on variousalgorithms. One embodiment utilizes a “least recently used” algorithm,in which the register file bypass controller 365 uses the least recentlyaccessed location. In another embodiment, the register file bypasscontroller 365 tracks which locations have been read from since data waslast loaded into them, and replaces locations that have been read atleast once. After determining the location, the register file bypasscontroller 365 writes the data into the chosen location. Once the datais written, the register file bypass controller 365 sets the multiplexer363 to select the input coupled to the correct location. The executionunit then receives the data from the correct location, and executes theinstruction.

FIG. 4 depicts a flowchart of example operations for determining ifinstructions in the IQ reference a cast out register tag. FIG. 4presents operations in an example order from which embodiments candeviate (e.g., operations can be performed in a different order thanillustrated and/or in parallel).

At block 401, an IQ receives notification from a register filecontroller that data was cast out from a physical register in the FLRF.The register file controller sends the register tag identifying thephysical register as the notification. Control then flows to block 403.

At block 403, the IQ begins a loop during which it determines which, ifany, instructions in the IQ reference the register tag identifying thephysical register from which data was cast out. The IQ initializes theloop by setting a pointer to the first instruction. On subsequent loops,the IQ sets the pointer to the next instruction. The instruction thepointer references is referred to as the “current instruction.” Controlthen flows to block 405.

At block 405, the IQ determines if the current instruction referencesthe register tag associated with the cast out data. This is accomplishedby comparing the register tag to any register tags associated with thecurrent instruction operands (instruction register tags). If one or moreinstruction register tags match, control then flows to block 407. If noinstruction register tags match, control then flows to block 409.

At block 407, the IQ sets the “needs_rtag” flag for any instructionregister tag found by the IQ at block 405. The “needs_rtag” flag may bea single bit associated with the instruction or a special value that issubstituted in place of a valid register tag. The “needs_rtag” flagindicates that the instruction consumes the data that has been cast outof the physical register identified by the register tag or RTAG. Controlthen flows to block 409.

At block 409, the IQ determines if all instructions in the IQ have beenchecked at block 405. If unchecked instructions exist in the IQ, controlthen loops back to block 403. If no unchecked instructions exist, theloop and process end.

As noted above the “needs_rtag” flag indicates that an instructionconsumes data that was cast out from the FLRF. Before an instructionissues from the IQ, the IQ determines whether the instruction consumesdata cast out from the FLRF based on the “needs_rtag” flag. It will beappreciated that the name of the flag and technique for implementing theflag can vary. If the flag has been set for the instruction, then the IQsends a register bypass request to a register file bypass controller. Insome embodiments, the IQ comprises one or more components to implementfunctionality of a register bypass controller. In such cases, a requestis not necessarily sent. The IQ can send the instruction to otherinternal components to perform the operations for register bypassresponsive to determining that the flag has been set for theinstruction.

FIG. 5 depicts a flowchart of example operations for utilizing aregister file bypass. FIG. 5 presents operations in an example orderfrom which embodiments can deviate (e.g., operations can be performed ina different order than illustrated and/or in parallel).

At block 503, a register file bypass controller receives a register filebypass request from an IQ. The register file bypass request includes aninstruction identifier, the thread identifier associated with theinstruction and the logical register identifiers of any operands thatare register-bypass eligible. An operand is register-bypass eligible ifa swap request is not pending for that particular operand and theoperand data is not in the FLRF. Control then flows to block 505.

At block 505, the register file bypass controller determines if theregister file bypass request should be granted or denied. The grantingor denying of a request is based on multiple factors, many of which aredependent on the hardware architecture design. For example, someembodiments may use dedicated bypass registers in the FLRF. These bypassregisters may share SLRF read ports with the other physical registers inthe FLRF. The sharing of SLRF read ports means that register file bypassrequests and swap requests from the register file mapper compete for useof the ports. In these embodiments, the register file bypass controllermay limit the number of bypass requests in order to allow swap requeststo use the SLRF read ports, thus allowing swap requests to complete. Inembodiments where the bypass registers have a dedicated SLRF read port,the register file bypass controller can grant more register file bypassrequests because there will be no conflicts with swap requests.Additionally, the number of register file bypass requests granted willdepend on the number of storage locations available for register filebypass requests. If a smaller number of storage locations exist, thenfewer register file bypass requests can be granted than if a largenumber of storage locations exist. If the register file bypasscontroller determines the register file bypass request should begranted, control then flows to block 507. If the register file bypasscontroller determines the register file bypass request should be denied,control then flows to block 521.

At block 507, the register file bypass controller determines if the datafor which the register file bypass request was granted already exists inone of the register file bypass storage locations. The register filebypass controller tracks the thread identifier and logical registeridentifiers associated with the data in the register file bypass storagelocations. The register file bypass controller compares the threadidentifier and logical register identifier sent with the register filebypass request to the thread and logical register identifiers associatedwith the data in the register file bypass storage locations. If the datafor which the register file bypass request was granted already does notexist in one of the register file bypass storage locations, control thenflows to block 509. If the data for which the register file bypassrequest was granted already exists in one of the register file bypassstorage locations, then control then flows to block 513.

At block 509, the register file bypass controller determines whichregister file bypass location to write the data into. The location canbe determined by a variety of different algorithms. For example, theregister file bypass controller can track the number of cycles sinceeach location was accessed, referred to as the “age.” The register filebypass controller can then be selected based on a “least recently used”algorithm. The data would then be loaded into the storage location thatis the oldest. The register file bypass controller can also track whichstorage locations have been accessed since data was last loaded into thestorage locations. The register file bypass controller can then assignstorage locations based on which data has already been read by theinstruction associated with that data. Control then flows to block 511.

At block 511, the register file bypass controller writes the data fromthe SLRF into the bypass storage location selected at block 509. Controlthen flows to block 521.

If the register file bypass controller determines that the requesteddata was already loaded into a register file bypass storage location atblock 507, then control flowed to block 513. At block 513, the registerfile controller sets the multiplexer to select the input coupled withthe correct register file bypass storage location. If control flowedfrom block 511, the input selected is the register file bypass storagelocation determined by the register file bypass controller at block 509.If control flowed from block 507, the input selected is the registerfile bypass storage location found by the register file bypasscontroller to contain the data at block 507.

If a register file bypass request was denied by the register file bypasscontroller at block 505 or after data was loaded into a bypass registerat block 511, control flowed to block 521. At block 521, the registerfile bypass controller notifies the appropriate execution unit to cancelexecution of the instruction for which the register file bypass wasrequested. In addition, the IQ or components of the IQ are notified thatexecution of the instruction has been cancelled. Execution of theinstructions is cancelled because the length of time it takes to loaddata into a bypass storage location is longer than the length of time ittakes for the instruction to reach the execution unit. Cancellation ofthe instruction allows the instruction to be reissued at a later timeafter the data is loaded into a bypass register location.

Efficient usage of a register file mapper is not limited to efficientusage of the FLRF alone. Inefficient usage of the mapping structure canlead to inefficient usage of the FLRF. Thus, efficient usage of themapping structure itself can lead to better performance as well. Forexample, the register file mapper mapping structure can be implementedsuch that each entry in the structure is indexed by the logical registeridentifier. Each entry can then have space for multiple threadidentifier register tag mappings. The size of each entry is determinedby the underlying hardware architecture, based on multiple factors.These factors include the amount of space available on the processor andpower consumption goals. The size of each entry determines the number ofthreads that can have mappings for a particular logical register at onetime.

Due to the large number of threads handled by a processor at once,entries may not be able to hold mappings for all threads. For example,assume entries in the map table are constrained to holding mappings forthree threads. If three threads already have mappings from logicalregister R0 to three separate physical registers, an attempt by a fourththread to use R0 would force one of the three existing mappings to beoverwritten. When a mapping is overwritten, the associated data is castout to the SLRF. Table 1 illustrates the map table for this scenarioprior to the fourth thread using R0.

TABLE 1 Index Mapping #1 Mapping #2 Mapping #3 LRID TID RTAG TID RTAGTID RTAG 0 1 01100 2 10011 3 11011 1 — — — — — — 2 — — — — — —

This is further impacted by register usage patterns. Most applicationsare written in a high level programming language, such as C++. Acompiler transforms (“compiles”) the high level programming languageinto the lower level instructions and machine code used by a processor.During this process, the compiler assigns logical registers to storevalues. The compiler frequently assigns logical registers in ascendingor descending numerical order. For example, if there are thirty-tworegisters, a compiler may assign R1 first, then R2, R3, etc., or beginwith R32, then R31, R30, and so on. Compilers also reuse previousregisters when possible. Because of these two factors, the beginningregisters (R1, R2 . . . ) or the ending registers (R32, R31 . . . ) tendto be used more often depending upon the assignment paradigm. In aprocessor that is executing multiple threads, this may lead to a higherdemand for certain registers, and a lower demand for others. Whencombined with the mapping structure space constraints, this leads toincreased conflicts for certain entries in the mapping structure, whilesome entries are rarely used.

FIG. 8 depicts an example register usage pattern graph (hereinafter“graph”). The x-axis of the graph 800 represents the logical registeridentifiers and the y-axis of the graph 800 represents the number oftimes a logical register identifier is referenced. The solid line of thegraph 800 depicts actual register usage and the dotted line of the graph800 represents optimal register usage.

A register file mapper can be designed to create a mapping betweenlogical registers and physical registers by hashing the threadidentifier with the logical register identifier. Consider thehypothetical above where map table entries are constrained to threedifferent thread mappings per logical register identifier. A registerfile mapper that combines the thread identifier with the logicalregister identifier could produce the result illustrated in Table 2. Themap table is indexed by the combination of the thread identifier andlogical register identifier, defined as H(LRID, TID). Instead of allthree mappings existing in the first row, one mapping would be in eachrow. Thus, when a fourth thread references R0, no existing mapping isoverwritten, regardless of which row the entry is mapped to. In thegraph 800, this is equivalent to translating some references fromlogical register zero to logical register sixteen, as identified by thesolid black dots.

TABLE 2 Index Mapping #1 Mapping #2 Mapping #3 H(LRID, TID) TID RTAG TIDRTAG TID RTAG 0 1 01100 — — — — 1 2 10011 — — — — 2 3 11011 — — — —

FIG. 6 depicts the states of an example register file mapper implementedusing a hasher after a sequence of instructions. FIG. 6 depicts thestate 600 after a first example instruction 633 and the state 690 aftera second example instruction 635. The first example instruction 633 isassociated with thread 0 637 and the second example instruction isassociated with thread 1 639. Both states include a register file mapper620, an FLRF 640, and an SLRF 650. The register file mapper 620 includesa map table 621 and a hasher 631. The map table 621 is indexed bylogical register identifier 623, and contains thread identifiers 625 andregister tags 627. The FLRF 640 contains physical registers 641,identified by register tags. Arrow 680 represents the passage of time.In this example embodiment, thread identifiers and logical registeridentifiers are represented in binary using five bits and register tagsare six bits. Each entry in the map table 621 is limited to a mappingfor one thread at a time.

State 600 depicts the state of the system after the first instruction633 has dispatched, issued, and completed execution. The firstinstruction 633 instructs the execution unit to add the values containedin logical register zero and logical register two and place the resultinto logical register one. The register file mapper 620 receivesnotification that the instruction 633 completed execution, including thelogical destination register identifier and thread identifiers. Theregister file mapper 620 generates a hash of the logical destinationregister identifier and instruction thread identifier using the hasher631. The register file mapper 620 uses the hash value as the logicalregister identifier index into the map table 621 and provides the mappedregister tag to the execution unit. The execution unit writes the resultof the instruction into the physical register identified by the mappedregister tag 627. In some embodiments, the hasher 631 reverses the bitrepresentation of the thread identifier and does a bitwise exclusive-oroperation using the reversed thread identifier and logical destinationidentifier. The result of the bitwise exclusive-or operation applied tothe reversed thread identifier (00000) and logical destination registeridentifier (00001) is a hash value of 00001. The register file mapper620 selects the row 622 indexed by the hashed value 00001 from the maptable 621. The register file mapper 620 provides the register tag 011001to the execution unit. The execution unit writes the result of theoperation (R0+R2) into the physical register 641 identified by registertag 011001. This results in a single value stored in the FLRF 640,indicated by label 642.

State 690 depicts the state of the system after instruction 635 hasdispatched, issued, and completed execution. The second instruction 635instructs the execution unit to subtract the value in logical registertwo from the value in logical register zero and place the result intological register one. The register file mapper 620 receives notificationthat the instruction 635 completed execution, including the logicaldestination register identifier and thread identifiers. The registerfile mapper 620 generates a hash of the logical destination registeridentifier and instruction thread identifier using the hasher 631. Theregister file mapper 620 uses the hash value as the logical registeridentifier index into the map table 621 and provides the mapped registertag to the execution unit. The execution unit writes the result of theinstruction into the physical register identified by the mapped registertag 627. The hasher 631 reverses the bit representation of the threadidentifier and does a bitwise exclusive-or operation using the reversedthread identifier and logical destination identifier. The result of thebitwise exclusive-or operation applied to the reversed thread identifier(10000) and logical destination register identifier (00001) is a hashvalue of 10001. The register file mapper 620 selects the row 624 indexedby the hash value 10001 from the map table. The register file mapper 620provides the register tag 100111 to the execution unit. The executionunit writes the result of the operation (R0−R2) into the physicalregister 641 identified by register tag 100111. This results in a secondvalue being stored in the FLRF 640, indicated by label 644.

Without the hasher 631, the register file mapper 620 would have mappedthe logical destination register used by the second instruction to thesame entry as the logical destination register referenced by the firstinstruction. Due to the limitation of only one mapping per entry, theregister file mapper 620 would have replaced the existing mapping. Thisreplacement would have resulted in the register file mapper 620 castingout the first value 642 to the SLRF 650. If the value 642 is located inthe SLRF 650 and is referenced by a subsequent instruction, it may needto be swapped into the FLRF 640. Even if the embodiment utilizes aregister file bypass system as described above, the increased latencyresults in lower performance, and may increase contention for theregister file bypass data pathways.

Although not discussed above or depicted in the figures, there are manyalternative embodiments possible. The following alternatives andvariations do not constitute an exclusive list, and a specificembodiment may use any combination of the various embodiments discussed.Regarding flowcharts, not all operations illustrated and discussed arerequired. Additionally, other operations may be added to the processesdepicted.

In reference to FIG. 1, the components may be arranged in a variety ofconfigurations. For example, the IDU 103 can be embodied as part of theIFU 101. All components contained in the register file mapper 102 can beembodied as individual components or combined in various combinations.The register files can be embodied with multiple FLRFs 140 and multipleSLRFs 150. Additional levels of register files can be added, and theFLRF 140 can contain multiple individual register files. Instructionmetadata can take a variety of forms as well. The instruction may havebits reserved as flags, or registers may be designated to hold valuesthat act as flags or otherwise provide information about theinstruction. Special opcodes may be used either by a programmer or theprocessor. An example of the latter is the processor breaking down aninstruction into a microcoded routine, which can be a set ofinstructions using opcodes specific to microcoded routines. Theprocessor or register file controller 129 can generate and storemetadata and statistics about instructions, threads or the state of theprocessor or register file controller 129. This can be accomplished byusing a variety of methods, including counter registers and rate counterregisters. Statistics and similar metadata may be passed to theprocessor and register file controller 129 in a manner similar to usinga flag. Because threads and instructions are intrinsically linked,metadata about a thread can be considered metadata about the associatedinstructions.

Additionally, functionality of the components may be combined or varyfrom the described functionality. For example, the functions of thein-flight register mapper 110 and the completed register file mapper 102can be combined into one mapper serving both purposes. Execution units107 could write directly to the SLRF 150 or another register fileinstead of writing to the FLRF 140 and having the register filecontroller 129 cast the data out. Additionally, when the register filecontroller 129 casts data out, the physical register in the FLRF 140that held the data can be zeroed out or left untouched untiloverwritten.

In reference to FIG. 3, the components may be arranged in a variety ofconfigurations. For example, the latches 361 may be connected in aseries, in parallel, or a combination of both. Each of the executionunits 306 may have an individual set of latches 307, or a single set oflatches 307 may be shared between multiple execution units 306.Additionally, the number of latches 361 can vary between embodiments.The embodiments are not limited to using only latches 361 or only bypassregisters 347, but can use a combination of both. In an embodiment usingbypass registers 347, the bypass registers 347 may have a separate SLRFread port 375. This adds complexity to the design, but prevents registerfile bypass requests from conflicting with reads from the SLRF 350.

Additionally, functionality of the components may be combined or varyfrom the described functionality. For example, a component other thanthe register file bypass controller 365 may track the informationidentifying what data is in which register file bypass storage location.Additionally, the register file bypass controller 347 arbitration logicmay vary based on the design of the hardware. For example, when thebypass registers 347 share an SLRF read port 373 with the other physicalregisters 341, limits may be placed on the number of times a thread maybe granted a bypass request within a given period. In an embodiment thatdoes not share the SLRF read port 373 with the other physical registers341, this limit may be removed. Additionally, an alternative embodimentmay add the ability to request a swap for an instruction in the IQ 305.

In reference to FIG. 6, the use of a hasher 631 may be extended toadditional register files, such as the SLRF 650, to spread registerusage across banks of registers. Register bank usage patterns mayresemble register use patterns, as illustrated in FIG. 8. Thus, a hashmay be created using an instruction thread identifier or otherinformation and used to specify which register bank to write out to. Thehasher 631 can be embodied to operate on bits in parallel or serially.Additionally, the specific order of operations may differ in an actualimplementation, and more may be included. For example, the register tagidentifying the destination register for an instruction may be providedto the execution unit with the instruction, instead of after completion.

As will be appreciated by one skilled in the art, aspects of the presentinventive subject matter may be embodied as a system, method or computerprogram product. Accordingly, aspects of the present inventive subjectmatter may take the form of an entirely hardware embodiment, an entirelysoftware embodiment (including firmware, resident software, micro-code,etc.) or an embodiment combining software and hardware aspects that mayall generally be referred to herein as a “circuit,” “module” or“system.” Furthermore, aspects of the present inventive subject mattermay take the form of a computer program product embodied in one or morecomputer readable medium(s) having computer readable program codeembodied thereon.

Any combination of one or more computer readable medium(s) may beutilized. The computer readable medium may be a computer readable signalmedium or a computer readable storage medium. A computer readablestorage medium may be, for example, but not limited to, an electronic,magnetic, optical, electromagnetic, infrared, or semiconductor system,apparatus, or device, or any suitable combination of the foregoing. Morespecific examples (a non-exhaustive list) of the computer readablestorage medium would include the following: an electrical connectionhaving one or more wires, a portable computer diskette, a hard disk, arandom access memory (RAM), a read-only memory (ROM), an erasableprogrammable read-only memory (EPROM or Flash memory), an optical fiber,a portable compact disc read-only memory (CD-ROM), an optical storagedevice, a magnetic storage device, or any suitable combination of theforegoing. In the context of this document, a computer readable storagemedium may be any tangible medium that can contain, or store a programfor use by or in connection with an instruction execution system,apparatus, or device.

A computer readable signal medium may include a propagated data signalwith computer readable program code embodied therein, for example, inbaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including, but not limited to,electro-magnetic, optical, or any suitable combination thereof. Acomputer readable signal medium may be any computer readable medium thatis not a computer readable storage medium and that can communicate,propagate, or transport a program for use by or in connection with aninstruction execution system, apparatus, or device.

Program code embodied on a computer readable medium may be transmittedusing any appropriate medium, including but not limited to wireless,wireline, optical fiber cable, RF, etc., or any suitable combination ofthe foregoing.

Computer program code for carrying out operations for aspects of thepresent inventive subject matter may be written in any combination ofone or more programming languages, including an object orientedprogramming language such as Java, Smalltalk, C++ or the like andconventional procedural programming languages, such as the “C”programming language or similar programming languages. The program codemay execute entirely on the user's computer, partly on the user'scomputer, as a stand-alone software package, partly on the user'scomputer and partly on a remote computer or entirely on the remotecomputer or server. In the latter scenario, the remote computer may beconnected to the user's computer through any type of network, includinga local area network (LAN) or a wide area network (WAN), or theconnection may be made to an external computer (for example, through theInternet using an Internet Service Provider).

Aspects of the present inventive subject matter are described withreference to flowchart illustrations and/or block diagrams of methods,apparatus (systems) and computer program products according toembodiments of the inventive subject matter. It will be understood thateach block of the flowchart illustrations and/or block diagrams, andcombinations of blocks in the flowchart illustrations and/or blockdiagrams, can be implemented by computer program instructions. Thesecomputer program instructions may be provided to a processor of ageneral purpose computer, special purpose computer, or otherprogrammable data processing apparatus to produce a machine, such thatthe instructions, which execute via the processor of the computer orother programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

These computer program instructions may also be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

FIG. 7 depicts an example computer system with a performance-based levelselecting register file controller. A computer system includes aprocessor unit 701 (possibly including multiple processors, multiplecores, multiple nodes, and/or implementing multi-threading, etc.). Thecomputer system includes memory 703. The memory 703 may be system memory(e.g., one or more of cache, SRAM, DRAM, zero capacitor RAM, TwinTransistor RAM, eDRAM, EDO RAM, DDR RAM, EEPROM, NRAM, RRAM, SONOS,PRAM, etc.) or any one or more of the above already described possiblerealizations of machine-readable media. The computer system alsoincludes a bus 711 (e.g., PCI, ISA, PCI-Express, HyperTransport® bus,InfiniBand® bus, NuBus bus, etc.), I/O devices 709 (e.g., keyboard,mouse, monitor, microphone, speaker, etc.), a network interface 707(e.g., an ATM interface, an Ethernet interface, a Frame Relay interface,SONET interface, wireless interface, etc.), a cache 717 (e.g., a directmapped cache, a 2-way set associative cache, a fully associative cache,etc.), a register file mapper 721, a first level register file 723, asecond level register file 725 and a storage device(s) 713 (e.g.,optical storage, magnetic storage, etc.). The cache 717 may be a lowerlevel cache (e.g., L1 cache embodied in a processor) or a higher levelcache (e.g., L2 cache, L3 cache, etc.). The register file mapper 721,first level register file 723, and second level register file 725 embodyfunctionality to implement embodiments described above. The registerfile mapper 721 functions as described above, utilizing aperformance-based level selecting register file controller 727 toefficiently make use of the first level register file 723 and the secondlevel register file 725. The register file mapper 721 can also utilize aregister file bypass system and hasher to further improve the efficiencyof the first level register file 723 and the second level register file725. Any one of these functionalities may be partially (or entirely)implemented in hardware and/or on the processing unit 701. For example,the functionality may be implemented with an application specificintegrated circuit, in logic implemented in the processing unit 701, ina co-processor on a peripheral device or card, etc. Further,realizations may include fewer or additional components not illustratedin FIG. 7 (e.g., video cards, audio cards, additional networkinterfaces, peripheral devices, etc.). The processor unit 701, storagedevice(s) 713, network interface 707, cache 717, and I/O devices 709 arecoupled to the bus 711. Although illustrated as being coupled to the bus711, the memory 703 may be coupled to the processor unit 701.

While the embodiments are described with reference to variousimplementations and exploitations, it will be understood that theseembodiments are illustrative and that the scope of the inventive subjectmatter is not limited to them. In general, techniques for efficientlyutilizing a multi-level register file as described herein may beimplemented with facilities consistent with any hardware system orhardware systems. Many variations, modifications, additions, andimprovements are possible.

Plural instances may be provided for components, operations orstructures described herein as a single instance. Finally, boundariesbetween various components, operations and data stores are somewhatarbitrary, and particular operations are illustrated in the context ofspecific illustrative configurations. Other allocations of functionalityare envisioned and may fall within the scope of the inventive subjectmatter. In general, structures and functionality presented as separatecomponents in the exemplary configurations may be implemented as acombined structure or component. Similarly, structures and functionalitypresented as a single component may be implemented as separatecomponents. These and other variations, modifications, additions, andimprovements may fall within the scope of the inventive subject matter.

What is claimed is:
 1. A method comprising: receiving an indication thatparticular data has been selected for moving from a first level registerfile to a second level register file; determining that an unissuedinstruction references the particular data selected for moving from thefirst level register file to the second level register file; responsiveto said determining that the unissued instruction references theparticular data selected for moving from the first level register file,indicating the instruction references the particular data in the secondlevel register file; determining that a set of storage locations may beused to host cast out data from the second level register file, whereinthe set of storage locations are designated for bypassing registermapping; responsive to said determining that the set of storagelocations may be used to host cast out data from the second levelregister file, writing the particular data located in the second levelregister file to one of the set of storage locations; and indicatingthat the particular data referenced by the unissued instruction islocated in the one of the set of storage locations.
 2. The method ofclaim 1, further comprising: determining the one of the set of storagelocations will host the particular data in the second level registerfile; and responsive to said determining the one of the set of storagelocations will host the particular data in the second level registerfile, writing the particular data in the second level register file intothe one of the set of storage locations.
 3. The method of claim 2,wherein said determining the one of the set of storage locations willhost the particular data in the second level register file is based onat least one of how recently each storage location was used and whichstorage locations have been read from.
 4. The method of claim 1, whereinsaid determining that the set of storage locations may be used to hostthe cast out data from the second level register file is based, at leastpartly, on at least one of, availability of a second level register fileread port coupling the second level register file to the first levelregister file and status of a write from the first level register fileto the second level register file.
 5. The method of claim 1, wherein theindication that the particular data has been selected for moving fromthe first level register file to the second level register filecomprises at least one of the location of a physical register in thefirst level register file, and a thread identifier and a logicalregister identifier associated with the particular data that has beenselected for moving from the first level register file to the secondlevel register file.
 6. The method of claim 1, further comprising:requesting register mapping bypass for the particular data in the secondlevel register file responsive to said receiving the indication that theparticular data has been selected for moving from the first levelregister file to the second level register file and to determining thatthe unissued instruction references the particular data that has beenselected for moving from the first level register file to the secondlevel register file.
 7. The method of claim 6, wherein said requestingthe register mapping bypass for the particular data in the second levelregister file comprises an issue queue communicating a request thatindicates a thread identifier associated with the unissued instructionand a logical register identifier associated with the particular data inthe second level register file to a register mapping bypass controllerthat controls bypassing of register mapping of cast out data associatedwith logical registers that have been unmapped from physical registers.8. The method of claim 1, further comprising tracking the particulardata hosted in the set of storage locations and corresponding threads.9. The method of claim 1 further comprising: controlling a multiplexorto select input from the one of the set of storage locations, whereininputs of the multiplexor are coupled with the set of storage locationsand the first level register file and output of the multiplexor iscoupled with an execution unit.
 10. The method of claim 1, wherein saidindicating that the particular data referenced by the unissuedinstruction is located in the one of the set of storage locationscomprises updating the unissued instruction to reference the one of theset of storage locations.
 11. The method of claim 1, wherein the set ofstorage locations comprise at least one of physical registers in thefirst level register file designated as bypass locations and a set ofone or more latches coupled between an execution unit and the secondlevel register file.
 12. A method comprising: determining that a logicalregister has been selected for unmapping from a physical register in afirst level register file, wherein data in the physical register in thefirst level register file is moved to a physical register in a secondlevel register file; determining that an instruction in an issue queueindicates the logical register; loading the data into a first of a setof one or more storage locations designated for bypassing registermapping; and selecting the first of the set of one or more storagelocations as input to an execution unit that will execute theinstruction, wherein the execution unit executes the instruction withthe data while bypassing register mapping of the logical register. 13.The method of claim 12 further comprising determining that the data hasbeen loaded into the first of the set of one or more storage locationsdesignated for bypassing register mapping, wherein said selecting thefirst of the set of one or more storage locations as input to theexecution unit is after said determining that the data has been loadedinto the first of the set of one or more storage locations designatedfor bypassing register mapping.
 14. The method of claim 12 furthercomprising cancelling execution of the instruction to allow said loadingthe data into the first of the set of one or more storage locationsdesignated for bypassing register mapping, wherein the instruction wasissued prior to said loading the data into the first of the set of oneor more storage locations designated for bypassing register mapping. 15.The method of claim 12 further comprising: marking each of a pluralityof instructions in the issue queue that indicates the logical registerwith an indication of bypassing register mapping responsive to saiddetermining that the logical register has been selected for unmappingfrom the physical register in the first level register file; examiningeach instruction upon issuance for the indication of bypassing registermapping.
 16. A computer program product for bypassing a first levelregister file, the computer program product comprising: a non-transitorycomputer readable storage medium having computer usable program codeembodied therewith, the computer usable program code configured to:determine that a logical register has been selected for unmapping from aphysical register in a first level register file, wherein data in thephysical register in the first level register file is moved to aphysical register in a second level register file; determine that aninstruction in an issue queue indicates the logical register; load thedata into a first of a set of one or more storage locations designatedfor bypassing register mapping; and select the first of the set of oneor more storage locations as input to an execution unit that willexecute the instruction, wherein the execution unit executes theinstruction with the data while bypassing register mapping of thelogical register.
 17. The computer program product of claim 16 whereinthe computer usable program code embodied on the non-transitory computerreadable storage medium is further configured to determine that the datahas been loaded into the first of the set of one or more storagelocations designated for bypassing register mapping, wherein saidselecting the first of the set of one or more storage locations as inputto the execution unit is after said determining that the data has beenloaded into the first of the set of one or more storage locationsdesignated for bypassing register mapping.
 18. The computer programproduct of claim 16 wherein the computer usable program code embodied onthe non-transitory computer readable storage medium is furtherconfigured to cancel execution of the instruction to allow said loadingthe data into the first of the set of one or more storage locationsdesignated for bypassing register mapping, wherein the instruction wasissued prior to said loading the data into the first of the set of oneor more storage locations designated for bypassing register mapping. 19.The computer program product of claim 16 wherein the computer usableprogram code embodied on the non-transitory computer readable storagemedium is further configured to: mark each of a plurality ofinstructions in the issue queue that indicates the logical register withan indication of bypassing register mapping responsive to saiddetermining that the logical register has been selected for unmappingfrom the physical register in the first level register file; and examineeach instruction upon issuance for the indication of bypassing registermapping.
 20. A processor comprising: an execution unit; a first levelregister file of first physical registers; a second level register fileof second physical registers, wherein the first level register file ismore efficiently accessible relative to the second level register file;a plurality of storage locations designated to host data associated witha plurality of logical registers that will bypass register mapping; anda register file bypass controller coupled with the execution unit andthe second level register file, the register file bypass controlleroperable to determine an instruction that indicates a first logicalregister unmapped from one of the first physical registers in the firstlevel register file before the instruction issues, to load data of acorresponding logical register into one of the plurality of storagelocations, and to select one of the plurality of storage locations asinput to the execution unit for execution of the instruction whileavoiding mapping the corresponding logical register to the one of thefirst physical registers in the first level register file.
 21. Theprocessor of claim 20, wherein the plurality of storage locationscomprises at least one of the first physical registers in the firstlevel register file that are unavailable for register mapping and aplurality of latches.
 22. The processor of claim 20 further comprising amultiplexor coupled for input from the plurality of storage locationsand the first level register file, coupled to output to the executionunit, and coupled for control by the register file bypass controller.23. The processor of claim 20 further comprising an issue queue, whereinthe register bypass controller is coupled with the issue queue.
 24. Theprocessor of claim 20, wherein the register file bypass controller isfurther operable to track data loaded into the plurality of storagelocations and corresponding thread.